This invention relates to spread-spectrum communications in a code-division-multiple-access system, and more particularly to using side information from noise samples at an output from a matched filter at times other than a sampling time for a symbol sample, in a forward error correction decoder.
In a direct-sequence (DS) code-division-multiple-access (CDMA) system having a base station and a plurality of remote stations transmitting to the base station, the spread-spectrum signals from many of the remote stations arrive at the base station simultaneously. The spread-spectrum signal from each remote station may arrive at the base station with a different power level with different symbol and chip arrival times. Further, the desired spread-spectrum signal at a particular spread-spectrum receiver receiving a particular spread-spectrum channel from a particular remote station, may be fading, and is, on occasion, not detectable, or has a high error rate.
Diversity coding, forward-error-correction (FEC) decoding, and interference cancellation are approaches to reducing the error rates. RAKE may be used to combine the strongest signal paths in a fading or multipath environment. These approaches do not, in general, take advantage of the unique noise environment of a DS-CDMA system, in which noise, on the average, is due to the multiple spread-spectrum signals from the plurality of remote stations.
A general object of the invention is to reduce error rate in a direct-sequence code-division-multiple-access (DS-CDMA) spread-spectrum system.
According to the present invention, as embodied and broadly described herein, an improvement to a spread-spectrum receiver at the base station in a direct-sequence code-division-multiple-access (DS-CDMA) system is provided. The DS-CDMA system has a plurality of spread-spectrum signals arriving from a plurality of remote stations, respectively. Each spread-spectrum signal in the plurality of spread-spectrum signals has a chip-sequence signal lasting a symbol time Ts. Each remote user may be operating at a different symbol time Tsi, where i is an index for the different symbol time. Each chip-sequence signal is different, due to a different chip sequence, from other chip-sequence signals used by other spread-spectrum signals in the plurality of spread-spectrum signals.
Each spread-spectrum receiver at the base station includes a matched filter having an impulse response matched to a desired chip-sequence signal in the plurality of chip-sequence signals. The matched filter detects a desired spread-spectrum signal in the plurality of spread-spectrum signals arriving at the spread-spectrum receiver at the base station. The desired spread-spectrum signal is spread-spectrum processed with a desired chip-sequence signal. The desired spread-spectrum signal is the particular spread-spectrum signal for which data detection is sought.
The improvement comprises a symbol sampler, a noise sampler, an estimator or a low-pass filter, a combiner circuit, a magnitude device, a comparator and an erasure decoder. The symbol sampler samples at a plurality of symbol times nTs, a plurality of symbol samples from the desired matched filter. The integer n indexes the plurality of symbol times. Each symbol sample has time duration Ts.
For each symbol sample, the noise sampler samples at a plurality of chip times kTc, but not at the plurality of symbol times nTs, a plurality of noise samples from the matched filter. A plurality of noise samples is associated, or correspond, with each symbol sample. The estimator estimates a noise estimate from the plurality of noise samples. The low-pass filter filters the plurality of noise samples from the noise sampler, to generate an estimate of the noise. Each estimtate of the noise is denoted hereinafter as xe2x80x9cnoise estimatexe2x80x9d. The combiner circuit subtracts the noise estimated outputted from the low-pass filter, from the symbol sample, outputted from the symbol sampler, thereby generating a comparison signal. The magnitude device determines a magnitude of the comparison signal.
The comparator has a threshold voltage applied to a threshold input. The comparator compares the magnitude of the comparison signal to the threshold voltage. If the magnitude of the comparison signal fell below the threshold, then the comparartor outputs the erasure signal.
The erasure decoder decodes the symbols from the symbol sampler when the erasure signal is present from the comparator, as is well known in the art.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.